714 



SCIENCE 



[N. S. Vol. XXXVIII. No. 985 



stages, the seoond stage occurring at a tempera- 

 ture somewhat above — 33°. This may be attrib- 

 utable to the decomposition in successive stages of 

 the oxidizing agent used, or possibly to the for- 

 mation and subsequent decomposition of certain 

 complex hydronitrogens as unstable intermediate 

 products. 

 Fritz Feiedrichs: Critical Phenomena in Binary 



Systems. (Presented by A. W. Browne.) 

 Fritz Friedeichs, A. E. Houlehan and L. J. 

 Uleich: The System Ammonium Sulphate, Am- 

 monia. (Presented by A. W, Browne.) 

 Fritz Feiedrichs: The System Mercuric Chloride, 



Ammonia. (Presented by A. W. Browne.) 

 L. J. Ulrich: The System Ammonium Iodide, 



Ammonia. (Presented by A. W. Browne.) 

 G. J. FiNK: The System Ammonium Chloride. 



Ammo7iia. (Presented by A. W. Browne.) 

 G. J. Fink: The System Copper Sulphate, Am 



monia. (Presented by A. W. Browne.) 

 A. S. Yount: The System Silver Trinitride, Am 



monia. (Presented by A. W. Browne.) 

 J. W. Turrentine: The Structure of the Trinitride 

 Badicle. 



SYMPOSIUM ON photographic CHEMISTRY 



This symposium was held at Kodak Park. 

 Papers were presented as follows: 

 Geo. a. Peeley: The Production of Direct Photo- 

 graphic Positions. 

 P. G. Nutting: Practical Sensitometry. 



Photography sensitometry is the determination 

 of the relation between blackening and exposure. 

 Blackening is measured as density Z> = — log 

 transmission. Exposure is properly in ergs per 

 sq. cm. of a specific wave-length but in meter- 

 candle-seconds involving properties of the eye. 

 The Hurter and Drifiield curve, density against 

 log exposure gives the two chief characteristics — - 

 speed and contrast sensibility. Plates are fast or 

 slow, hard or soft working according to the shape 

 of this curve. 



Works tests are made by printing through a 

 tablet of gray and colored squares of graduated 

 density. , Laboratory tests are made by exposure 

 to a standard white light behind a rotating sec- 

 tored disk giving exposures of 1, 2, 4, 8 . . . 

 256, M-C-S. Densities are measured on a special 

 photometer. High precision sensitometry requires 

 many refinements of coating, exposure, develop- 

 ment, etc. 



S. E. Sheppaed: Some Applications of Quantita- 

 tive Absorption Spectroscopy in Chemistry. 

 Making use of the relations: 



(i) I^^^Ioa^i (Beer-Lambert law), where I A = in- 

 tensity of monochromatic light wave- 

 length X transmitted by an absorbing layer 

 of thickness d cm., and of concentration c 

 in grammes per liter, I = intensity of light 

 incident on same, a = a constant, the trans- 

 mission-coefficient, 

 (ii) M = C/a (Vierodt's equation), where M=: 

 molecular absorption ratio, 

 C = concentration in gram-molecules per 



liter, 

 o :^ transmission-coefficient of (i). 

 Then the absorption of light can be determined 

 quantitatively in regard to both color (wave-length 

 of light waves) and concentration of reacting 

 molecules. The principal applications considered 

 were as under : 

 (a) Analytical determination of amounts of dye- 



stufEs and colored salts in solutions. 

 (6) Technical: adjustment of ray -filters, 

 (c) Theoretical: application to problems of mo- 

 lecular constitution, of "solutions" and of 

 photo-chemical change. 

 L. A. Jones : Some Notes on the Cylindrical 

 Acetylene Flame a^ a Standard of Light. 

 A good reliable standard light source is a ne- 

 cessity in photographic sensitometry. The old- 

 style acetylene flame is not very satisfactory for 

 this purpose, on account of its sensitiveness to air 

 currents and the liability to parallax errors. A 

 newer type of standard acetylene burner designed 

 by Dr. Mees and Dr. Sheppard gives a cylindrical 

 flame much more steady and reliable than the flat 

 flame. 



Careful photometric measurements made on this 

 improved burner show that when properly adjusted 

 the intensity of light is constant even when the 

 gas pressure varies considerably. The results indi- 

 cate also that with proper care in construction, 

 especially in the width of slit used as screening dia- 

 phragm, different burners can be made that will 

 give the same light intensities to within 3 or 4 

 per cent. 



The investigation is not complete as yet, but un- 

 less unexpected difficulties arise, this form of 

 burner will undoubtedly be found very satisfactory 

 as a standard light source for sensitometric work. 

 Orim Tugman : The Sensitiveness Curves of Photo- 

 graphic Plates Exposed to X-Bays. 

 According to the equation given by Hurter and 

 Driffield for the relation between the exposure and 

 the development density in photographic plates the 

 density of a plate exposed to X-rays should be 

 directly proportional to exposure because the ea- 



